1. Field of the Invention
The present general inventive concept relates generally to a method and apparatus to evaluate a quality of an audio signal, and more particularly, to a method and apparatus to evaluate a quality of an audio signal according to a number of channels of the audio signal.
2. Description of the Related Art
Generally, an audio codec refers to a device to code and decode various audio signals, including a voice signal. A wide variety of multi-channel codecs have been recently developed. For example, a multi-channel codec, such as a 5.1-channel audio codec, is mainly used to code and decode audio signals in multimedia contents such as movies, and supports additional audio channels to give surround effects using rear speakers. Meanwhile, along with the increase in broadcast or Internet broadcast services over a wireless network, such as Internet Protocol TV (IPTV) and Digital Multimedia Broadcasting (DMB), there is an increasing demand for various voice codecs, and it is important to select an appropriate voice codec according to the purpose of the service.
In order to select an appropriate voice codec according to the type of the service, it is necessary to evaluate a quality of an audio signal that is output through a voice codec. For the audio quality evaluation, listening evaluation may be performed in which a plurality of listeners directly listen to audio signals. Since the listening evaluation takes a lot of time and cost, a method of evaluating an audio quality using an audio quality evaluation apparatus is generally used. For reference, International Telecommunication Union (ITU)-R has issued ITU-R Recommendation BS.1387, which is a recommendation on an audio quality evaluation method for audio codecs.
An audio signal may be roughly classified into a mono signal, a stereo signal and a multi-channel signal according to the number of channels included in the audio signal. A typical example of the multi-channel signal includes a 5.1-channel signal.
Factors to be considered in audio quality evaluation may vary according to the number of channels of an audio signal. For example, the level of noises contained in an audio signal, the difference in tone, and the sound rolling in the time domain are factors that may be involved in audio quality evaluation regardless of the type of the audio signal. On the other hand, spatial factors such as the sound stage, the position of the sound, and the width of the sound source are not considered in evaluation of the mono signal. However, the spatial factors should be considered in evaluation of the stereo signal, and during evaluation of the multi-channel signal, the spatial factors should be considered more compared to during the evaluation of the stereo signal. That is, while factors for evaluating auditory qualities to give the surround effects are not needed in audio quality evalukion for an audio codec supporting only the mono-channel signal, the factors for evaluating auditory qualities to give the surround effects are very important in audio quality evaluation for a multi-channel audio codec. Undesirably, however, the conventional audio quality evaluation apparatus should be changed in structure according to the type of the audio signal.
In addition, the conventional audio quality evaluation apparatus uses a similar scheme to the evaluation scheme for the stereo signal in evaluating the multi-channel signal. Therefore, in case of the multi-channel signal, a correlation between the evaluation result obtained by the audio quality evaluation apparatus and the listening evaluation actually evaluated by the people may fall undesirably, which means the poor performance of the audio quality evaluation apparatus.
Because the factors to be considered during audio quality evaluation vary according to the number of channels of the audio signal, it is very important to select an appropriate evaluation scheme depending on the number of channels of an audio signal in audio quality evaluation. For this, however, a user must inconveniently find out the number of channels of the audio signal before the audio quality evaluation. Therefore, there is a need to automatically check the number of channels of an audio signal during audio quality evaluation.
Furthermore, listeners may listen to audio signals with a headphone, or with a speaker. Therefore, a listening environment of the listeners should also be considered during audio quality evaluation of the audio signals. That is, a way of processing an output signal of an audio codec for audio quality evaluation of an audio signal should be changed according to the listening environment for the audio signal. However, the conventional audio quality evaluation apparatus has a poor audio quality evaluation performance because it does not consider these factors.